The present invention relates generally to proton exchange membrane (PEM) electrochemical cell stacks and relates more particularly to a novel PEM electrochemical cell stack.
In certain controlled environments, such as those found in airplanes, submarines and spacecrafts, it is often necessary for oxygen to be furnished in order to provide a habitable environment. An electrolysis cell, which uses electricity to convert water to hydrogen and oxygen, represents one type of device capable of producing quantities of oxygen. One common type of electrolysis cell comprises a proton exchange membrane, an anode positioned along one face of the proton exchange membrane, and a cathode positioned along the other face of the proton exchange membrane. To enhance electrolysis, a catalyst, such as platinum, is typically present both at the interface between the anode and the proton exchange membrane and at the interface between the cathode and the proton exchange membrane. The above-described combination of a proton exchange membrane, an anode, a cathode and associated catalysts is commonly referred to in the art as a membrane electrode assembly.
In use, water is delivered to the anode and an electric potential is applied across the two electrodes, thereby causing the electrolyzed water molecules to be converted into protons, electrons and oxygen atoms. The protons migrate through the proton exchange membrane and are reduced at the cathode to form molecular hydrogen. The oxygen atoms do not traverse the proton exchange membrane and, instead, form molecular oxygen at the anode. (An electrolysis cell, when operated in reverse to generate water and electricity using molecular hydrogen and molecular oxygen as starting materials, is referred to in the art as a fuel cell. Electrolysis cells and fuel cells both constitute electrochemical cells, and all discussion herein pertaining to electrolysis cells is correspondingly applicable to fuel cells.)
Often, a number of electrolysis cells are assembled together in order to meet hydrogen or oxygen production requirements. One common type of assembly is a stack comprising a plurality of stacked electrolysis cells that are electrically connected in series in a bipolar configuration. In one common type of stack, each cell includes, in addition to a membrane electrode assembly of the type described above, a pair of multi-layer metal screens, one of said screens being in contact with the outer face of the anode and the other of said screens being in contact with the outer face of the cathode. The screens are used to form the fluid cavities within a cell for the water, hydrogen and oxygen. Each cell additionally includes a pair of polysulfone cell frames, each cell frame peripherally surrounding a screen. The frames are used to peripherally contain the fluids and to conduct the fluids into and out of the screen cavities. Each cell further includes a pair of metal foil separators, one of said separators being positioned against the outer face of the anode screen and the other of said separators being positioned against the outer face of the cathode screen. The separators serve to axially contain the fluids on the active areas of the cell assembly. In addition, the separators and screens together serve to conduct electricity from the anode of one cell to the cathode of its adjacent cell. Plastic gaskets seal the outer faces of the cell frames to the metal separators, the inner faces of the cell frames being sealed to the proton exchange membrane.
Additional information relating to electrolysis cell stacks includes the following patents and publications, all of which are incorporated herein by reference: U.S. Pat. No. 6,057,053, inventor Gibb, issued May 2, 2000; U.S. Pat. No. 5,466,354, inventors Leonida et al., issued Nov. 14, 1995; U.S. Pat. No. 5,366,823, inventors Leonida et al., issued Nov. 22, 1994; U.S. Pat. No. 5,350,496, inventors Smith et al., issued Sep. 27, 1994; U.S. Pat. No. 5,324,565, inventors Leonida et al., issued Jun. 28, 1994; U.S. Pat. No. 5,316,644, inventors Titterington et al., issued May 31, 1994; U.S. Pat. No. 5,009,968, inventors Guthrie et al., issued Apr. 23, 1991; and Coker et al., xe2x80x9cIndustrial and Government Applications of SPE Fuel Cell and Electrolyzers,xe2x80x9d presented at The Case Western Symposium on xe2x80x9cMembranes and Ionic and Electronic Conducting Polymer,xe2x80x9d May 17-19, 1982 (Cleveland, Ohio).
In order to ensure optimal conversion of water to hydrogen and oxygen by each electrolysis cell in a stack, there must be uniform current distribution across the active areas of the electrodes of each cell, and there must be a proper sealing of cells to prevent the escape of fluids therefrom. Such uniform current distribution and proper sealing require that uniform contact pressure be applied to the cells while, at the same time, permitting movement of the cell components to compensate for thermal expansion and component creeps. These objectives are typically met by providing an electrically-conductive compression pad between adjacent cells in a stack and by compressing the cells of the stack between a top end plate and a bottom end plate. Compression between the two end plates is typically achieved by mounting both end plates on one or more xe2x80x9ctie rodsxe2x80x9d(i.e., posts), with one or both of said end plates being adapted for sliding movement on said xe2x80x9ctie rods,xe2x80x9d and by mounting springs, typically in the form of Belleville spring washers or the like, on the tie rods external to the end plates in such a way as to bias the end plates towards one another.
Because the amount of spring loading that is required for compression can be quite large in many instances (for example, where operating pressures are in the range of 200 to 6000 psi or where a large number of cells are in a stack), it is common to use stacks of Belleville spring washers on each tie rod, with multiple washers being stacked in parallel to increase load and being stacked in series to increase movement. However, as can readily be appreciated, the stacking of such springs external to the end plates adds to the overall height of the cell stack, a result that may, in some instances, be objectionable.
It is an object of the present invention to provide a novel PEM electrochemical cell stack.
It is another object of the present invention to provide a novel PEM electrochemical cell stack that overcomes at least some of the shortcomings described above in connection with existing PEM electrochemical cell stacks.
It is still another object of the present invention to provide a PEM electrochemical cell stack that has a compact design.
Therefore, in accordance with the foregoing objects and/or other objects to be described in or to become apparent from the description which follows, there is provided, according to one aspect of the invention, a proton exchange membrane (PEM) electrochemical cell stack comprising (a) a first sub-stack, said first sub-stack comprising a plurality of proton exchange membrane (PEM) electrochemical cells arranged in series in a bipolar configuration; (b) a second sub-stack, said second sub-stack comprising a plurality of proton exchange membrane (PEM) electrochemical cells arranged in series in a bipolar configuration; (c) a top end plate; (d) a bottom end plate; (e) a first intermediate support, said first sub-stack being stacked between said top end plate and said first intermediate support, said second sub-stack being stacked between said first intermediate support and said bottom end plate; (f) wherein said top end plate, said first intermediate support and said bottom end plate all extend beyond the peripheries of said first and second sub-stacks; (g) a first tie rod, said first tie rod being coupled to said top end plate and extending downwardly from said top end plate through said first intermediate support at a point peripheral to both of said first and second sub-stacks, said first tie rod terminating prior to said bottom end plate; and (h) first biasing means, mounted on said first tie rod below said first intermediate support and above said bottom end plate, for biasing said first intermediate support towards said top end plate.
The aforementioned intermediate support may be either a plate or an annular support. Where the intermediate support is a plate, electrical insulation is preferably additionally provided to electrically isolate the plate from the tie rods and/or biasing means. Such insulation may not be necessary for an annular support where the annular support does not extend radially inward to where the support is in contact with the electrically-conductive components of the sub-stack.
In a preferred embodiment, the electrochemical cell stack of the present invention comprises first and second sub-stacks of series-connected, proton exchange membrane (PEM) electrochemical cells. The first sub-stack is stacked between a top end plate and an intermediate plate, and the second sub-stack is stacked between the intermediate plate and a bottom end plate, the top end plate, the intermediate plate and the bottom end plate all extending beyond the peripheries of the first and second sub-stacks. A first set of tie rods is coupled to the top end plate and extends downwardly therefrom through the intermediate plate at points peripheral to the first and second sub-stacks, the first tie rods terminating prior to the bottom end plate. A Belleville washer spring stack is mounted on each of the first tie rods below the intermediate plate and above the bottom end plate for biasing the intermediate plate towards the top end plate. A second set of tie rods is coupled to the bottom end plate and extends upwardly therefrom through the intermediate plate at points peripheral to the first and second sub-stacks, the second tie rods terminating prior to the top end plate. A Belleville washer spring stack is mounted on each of the second tie rods above the intermediate plate and below the top end plate for biasing the intermediate plate towards the bottom end plate. The first and second sets of tie rods are preferably interlaced in an alternating pattern around the entire periphery of the sub-stacks.
As can readily be appreciated, because, in the electrochemical cell stack of the present invention, the Belleville spring washer stacks are positioned between the top and bottom end plates, the overall size of the cell stack is kept to a minimum. This is a considerable advantage over comparable existing cell stacks.
In addition, another particularly advantageous feature of the PEM electrochemical cell stack of the present invention is that the sub-stacks thereof can be electrically interconnected either in series or in parallel. By connecting the sub-stacks in parallel, the current capacity of the stack can be substantially increased while using a single set of end plates and the same compression hardware. Moreover, the current capacity can be further increased by introducing additional intermediate supports into the cell stack.